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Water supply may be sickener: alternative one seen (11)

Ross

I believe the answer is:

fluoridated

'water supply may be' is the definition.
(a water supply may be fluoridated)

'sickener alternative one seen' is the wordplay.
'sickener' becomes 'flu'.
'alternative' becomes 'or'.
'one' becomes 'i' (Roman numeral).
'seen' becomes 'dated' ('date' can be a synonym of 'see').
'flu'+'or'+'i'+'dated'='FLUORIDATED'

I've seen this clue in The Times.
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